Four tiny planets are orbiting one of our nearest stars - UNIVERSE

An artist's conception of the view from one of the four planets orbiting Barnard's Star. Credit: International Gemini Observatory / NOIRLab / NSF / AURA / R. Proctor / J. Pollard

Astronomers have revealed new evidence that not just one, but four tiny planets are circling around Barnard's Star, the second-nearest star system to Earth.

The four planets, each only about 20 to 30% the mass of Earth, are so close to their home star that they zip around the entire star in a matter of days. That probably means they are too hot to be habitable, but the find is a new benchmark for discovering smaller planets around nearby stars.

The resulting paper is published in The Astrophysical Journal Letters.

"It's a really exciting find—Barnard's Star is our cosmic neighbor, and yet we know so little about it," said Ritvik Basant, Ph.D student at the University of Chicago and first author on the study. "It's signaling a breakthrough with the precision of these new instruments from previous generations."

The finding adds weight to a November study by a team using a different telescope, which had found strong evidence for one planet around Barnard's Star and hints at others.

The new study included scientists with the Gemini Observatory/National Science Foundation NOIRLab, Heidelberg University, and the University of Amsterdam. 

An artist's conception of the four planets circling around Barnard's Star. The planets are so close to the star that they zip around their sun in a matter of days. Credit: International Gemini Observatory / NOIRLab / NSF / AURA / R. Proctor / J. Pollard

Star wobbles

For a century, astronomers have been studying Barnard's Star in hopes of finding planets around it. First discovered by E. E. Barnard at Yerkes Observatory in 1916, it is the nearest system that has the same configuration that we do—i.e., with only one star. (The absolute nearest star system to us, Proxima Centauri, has three stars circling each other, which changes the dynamics of planet formation and orbits).

Barnard's Star is a type called an M dwarf star, which we now know is extremely numerous in the universe. Scientists, therefore, would like to know more about what kinds of planets they host.

The trouble is that these faraway planets are far too tiny to be seen next to the brilliance of their stars, even with our most powerful telescopes. That means scientists have had to get creative to search for them.

One such effort was led by UChicago Prof. Jacob Bean, whose team created and installed an instrument called MAROON-X, which is attached to the Gemini Telescope on a Hawaiian mountaintop and designed specifically to search for distant planets.

Because stars are so much brighter than their planets, it's easier to look for effects that planets have on their stars—like monitoring the wind by watching how a flag moves.

MAROON-X looks for one such effect; the gravity of each planet tugs slightly on the star's position, meaning the star seems to wobble back and forth. MAROON-X measures the color of the light so precisely that it can pick up these minor shifts, and even tease apart the number and masses of the planets that must be circling the star to have this effect.

Basant, Bean, and the team rigorously calibrated and analyzed data taken during 112 different nights over a period of three years. They found solid evidence for three planets around Barnard's Star.

When the team combined their findings with data from the November experiment by a different team, which was taken by an instrument called ESPRESSO at the Very Large Telescope in Chile, they saw good evidence of a fourth planet.

These planets are likely rocky planets, rather than gas planets like Jupiter, the scientists said. That will be difficult to pin down with certainty; the angle we see them from Earth means we can't watch them cross in front of their star, which is the usual way to find out if a planet is rocky. But by gathering information about similar planets around other stars, we can make better guesses about their makeup.

However, the team was able to rule out, with a fair degree of certainty, the existence of other planets in the habitable zone around Barnard's Star.

'Really exciting'

Barnard's Star has been called the "great white whale" for planet hunters; several times over the past century, groups have announced evidence that suggested planets around Barnard's Star, only for them to be later disproven.

But these latest findings, independently confirmed in two different studies by the different instruments ESPRESSO and MAROON-X, mean a much larger degree of confidence than any previous result.

"We observed at different times of night on different days. They're in Chile; we're in Hawaii. Our teams didn't coordinate with each other at all," Basant said. "That gives us a lot of assurance that these aren't phantoms in the data."

These are among the smallest planets yet found with this observing technique. The scientists hope this will mark a new era of finding more and more planets in the universe.

Most of the rocky planets we've found so far are much larger than Earth, and they appear to be fairly similar across the galaxy. But there are reasons to think the smaller planets will have more widely varied compositions. As we find more of them, we can begin to tease out more information about how these planets form—and what makes planets likely to have habitable conditions.

The find itself was exciting, too, the scientists said.

"We worked on this data really intensely at the end of December, and I was thinking about it all the time," Bean said. "It was like, suddenly we know something that no one else does about the universe. We just couldn't wait to get this secret out.

"A lot of what we do can be incremental, and it's sometimes hard to see the bigger picture," he said. "But we found something that humanity will hopefully know forever. That sense of discovery is incredible."

Additional University of Chicago authors on the paper were postdoctoral fellows Rafael Luque, Lily L. Zhao, Tanya Das, and David Kasper; graduate student Madison Brady; postbaccalaureate student Nina Brown; and masters student Rohan Gupta.  

by Louise Lerner, University of Chicago

Source: Four tiny planets are orbiting one of our nearest stars